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I am trying to build a spectrometer to measure the color intensity of a solution.

By chemistry I can make up a solution that has exactly 1 mg of gold in it and another that has 5 mg of gold in it. If I extract each of these into 15ml of solvent I will get a solution of known color and volume. I will have two knowns, each with a different color intensity. These color intensities increase uniformly over the small increments taken and I can determine the exact weight of a similar prepared unknown sample, this is called a chemical assay.

I built my unit to measure this color intensity using a 2" solar cell from a solar pathway light, a multimeter, and a LED light source. It is all enclosed so the multimeter reads zero with the light off.

I have the multimeter set on 2000 mV. When I shine my LED light thru my sample solution vial I get a reading of 1600 mV for the 1 mg and 1400 mV for the 5mg. (Remember more = color less light.) That would be perfect, a 200 mV difference, or 50 mV per mg of gold.

When the light was held on the sample for a few seconds the readings started to go up and down one or two digits at a time and varied over 25 mV up and down. A ball breaker!

I tried a laser which did not work at all, and then tried a flash light with an incandescent bulb, did not work either. I also took the unit out into direct sun light and it still jumped around.

When I had the unit apart at my desk testing it with different lights, I noticed that my desk light was shining on it and it was giving me a stable reading. This lamp is a 60 watt incandescent soft light.

I also read where solar cells like halogen lights. I was trying to make this unit portable so I can do assays in the field so having a 60 watt, 110 volt light source is not practical. I was looking for other lighting (10 volt max) or different solar cell.

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    \$\begingroup\$ I think you mistyped: MV, is the abbreviation for megavolt, or 1,000,000 volts. I think you want Millivolts (mV). How stale is your solution or is it a suspension which I would not expect to be stable when not being agitated. Also it appears the light source angle etc was not consistent from test to test. \$\endgroup\$
    – Gil
    Jun 26 at 1:31
  • \$\begingroup\$ Worth pointing out: absorbance is exponential in concentration (see Beer's law), not linear, so the transmittance you measure will be the logarithm of the actual concentration. \$\endgroup\$ Jun 26 at 14:02
  • \$\begingroup\$ thank you for your comments tried 2 different diodes no change. light source fixed and solution true not a colloidal \$\endgroup\$
    – don myers
    Jun 26 at 17:47
  • \$\begingroup\$ meant millivolts used beers law but over 5 mg it is linear \$\endgroup\$
    – don myers
    Jun 26 at 17:52
  • \$\begingroup\$ The way to do this is with vis/it spectrometers, they are calibrated and use broadband sources and calibrated photodiodes to correctly measure materials \$\endgroup\$
    – Voltage Spike
    Jul 4 at 0:45
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You need to copy the design used in cheap LED pulse oximeters: use two light sources, in their case LEDs of different wavelengths which are absorbed differently. Measure the difference between the three values you get (light A, light B, neither) and you can cancel out the effects of ambient light, absorption by the container, etc.

You should select two wavelengths based on what LEDs you can find and the absorption spectrum of the target gold substance.

A photodiode may work better, but will require amplification with all the problems associated with that.

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    \$\begingroup\$ Whoever approved that edit, please think more before approving edits! \$\endgroup\$
    – Hearth
    Jul 4 at 1:42
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    \$\begingroup\$ @donmyers Use the comments, not an edit, if you have something to say. \$\endgroup\$
    – Hearth
    Jul 4 at 1:42
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Don't use a solar cell, use a photodiode. They're designed for measurement, and are not expensive. Measure the current produced by one, not the voltage, as this is more linear to the incoming light.

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